Sensitivity Assessment for Spaceborne Measurement of Atmospheric Column Carbon Dioxide Using Reflected Sunlight at 1.58 μm

摘要

A series of sensitivity studies is carried out to explore the feasibility of spaceborne global carbon dioxide (CO_2) measurements for global and regional carbon cycle studies. The detection method uses absorption of surface reflected sunlight in the CO_2 vibration-rotation band at 1.58 μm. The sensitivities of the detected radiances are calculated using the line-by-line model (LBLRTM), implemented with the DISORT (Discrete Ordinates Radiative Transfer) model for atmospheric scattering. The results indicate that (a) the small (～1% or 3-4 ppmv) changes in CO_2 near the Earth's surface are detectable in this CO_2 band provided adequate sensor signal-to-noise ratio and spectral resolution are achievable; (b) the radiance signal or sensitivity to CO_2 change near the surface is not significantly diminished even in the presence of aerosols and/or thin cirrus clouds in the atmosphere; (c) the modification of sunlight path length by scattering of aerosols and/or cirrus clouds could lead to large systematic errors in the retrieval; therefore, ancillary aerosol/cirrus cloud data are important to reduce retrieval errors; (d) CO_2 retrieval requires good knowledge of the atmospheric temperature profile, e.g. approximately 1K RMS error in layer temperature; (e) the atmospheric path length, over which the CO_2 absorption occurs, must be known in order to correctly interpret horizontal gradients of CO_2 from the column CO_2 measurement; thus an additional sensor for surface pressure measurement needs to be attached for a complete measurement package.